1. Field of the Invention
This invention relates to the formation of metal compound thin films by sputtering and more particularly, to means for effectively controlling the degree of oxidation, nitrogenation or carbonization of metal compounds in forming metal compound thin films by sputtering. It also relates to a method for preparing a rubber composite material having an improved bond between a substrate and a rubber layer.
2. Prior Art
Films of metal compounds are often formed by vacuum evaporation, ion plating and sputtering processes. The vacuum evaporation process is most common. Since a metal compound source is evaporated to deposit a film having the same composition as the source, it is impossible to control the degree of oxidation, nitrogenation or carbonization of the deposited film. The ion plating process involves mixing a rare gas during evaporation to ionize the source material prior to deposition of a film. By varying the ratio of ionizing rare gas to reactant gas, the degree of oxidation of the deposited film can be controlled, but to a limited extent. In order that the degree of oxidation be controlled over a wider range, the rate of deposition and other parameters should also be varied. Among the sputtering process, a high-frequency magnetron sputtering process is suitable for forming films of metal compounds. As in the ion plating, the ratio of rare gas to reactant gas for oxidation or the like must be varied in order to control the degree of oxidation. The degree of oxidation is controlled only in a narrow range and the rate of deposition is extremely slow. Also known is a reactive magnetron sputtering process wherein DC magnetron sputtering is carried out at a selected ratio of rare gas to reactant gas to control the degree of oxidation or the like. It is difficult to deposit films of metal oxide in a reproducible manner by merely controlling the gas ratio. The degree of oxidation can also be controlled by resorting to emission spectroscopy during film deposition although this process lacks reproducibility.
In the field of forming metal compound thin films by sputtering or the like, it is desired to have an efficient simple method capable of controlling the degree of oxidation, nitrogenation or carbonization of deposited films in a reproducible manner.
Metal-rubber composite materials as typified by vibration damping rubber are widely used in the art. A firm bond between metal and rubber is crucial.
Japanese Patent Application Kokai (JP-A) Nos. 87311/1987 and 246278/1987 by the same assignee as the present invention disclose a method for preparing a composite material comprising the steps of dry plating cobalt or cobalt alloy onto a substrate, applying unvulcanized rubber thereon, and vulcanizing the rubber to the substrate whereupon the rubber is firmly bonded to the substrate.
Also in JP-A 290342/1989, a cobalt coating is heat treated during or after deposition to oxidize the cobalt for the purpose of improving the durability (or resistance to wet heat deterioration) of a substrate/rubber composite. More particularly, for the purpose of improving wet heat resistance, reactive sputtering is carried out in argon gas in admixture with oxygen gas to form a cobalt oxide film. However, this method alone achieves insufficient oxidation, failing to fully improve resistance to wet heat deterioration. Resistance to wet heat deterioration can be improved by carrying out heat treatment after film formation to further oxidize the cobalt. Then two steps are necessary. This method failed to achieve a satisfactory bond in a follow-up test where reactive sputtering was carried out in another apparatus. It was thus difficult to carry out this method on a greater scale sufficient for commercial application.
A primary object of the invention is to provide a method for forming a thin film of metal compound, which ensures simple and reproducible control of the composition (such as a degree of oxidation) and physical properties of the film.
Another object of the invention is to provide a method for preparing a rubber composite material wherein rubber is bonded to a substrate through a cobalt oxide thin film, which method increases and controls the degree of oxidation of the cobalt oxide thin film in a simple manner so that the rubber is firmly bonded to the substrate and which method is applicable on a commercial scale.
Our discovery was derived from an experiment of forming a thin film of a metal compound on a surface of a substrate by sputtering, especially magnetron sputtering. A target of a corresponding metal or metal compound was placed in vacuum and in an atmosphere containing an inert gas and a reactive gas capable of reacting with atoms sputtered from the target to form the metal compound. The reactive gas was a gas having molecular oxygen when the metal compound is a metal oxide, a gas having molecular nitrogen when the metal compound is a metal nitride, and a gas having molecular carbon when the metal compound is a metal carbide. The inert gas and the reactive gas were mixed in a predetermined ratio. An input power was supplied to the target and an input voltage applied between the target and the substrate from a DC supply. The target was thus sputtered in the atmosphere at a predetermined vacuum. The input power and the input voltage were plotted on a graph. An abrupt change of the input voltage occurred at a certain input power. This input power is called a transition point. Quite unexpectedly, films of different performance, that is, having different compositions or physical properties were obtained depending on whether the input power with which sputtering was carried out was above or below the transition point. By adjusting a shift of input power from the transition point, a thin film of metal compound having a controlled composition (such as a controlled degree of oxidation) or controlled physical properties could be formed in a well reproducible manner.
We have also found an effective control means in the process of preparing a rubber composite material by forming a thin film of cobalt oxide on a surface of a substrate, forming a rubber coating on the thin film, and vulcanizing the rubber coating. When the cobalt oxide thin film is formed, a target of cobalt is sputtered in an atmosphere containing an inert gas and a gas having molecular oxygen by supplying an input power to the target and applying an input voltage between the target and the substrate from a DC supply. We have found that an abrupt change of the input voltage occurs at a transition point of input power and that an improved cobalt oxide thin film is obtained by controlling the input power during sputtering to be not lower than the transition point.
More particularly, in the formation of a cobalt oxide film represented by CoOx on a substrate surface by sputtering, especially magnetron sputtering, a mixture of inert gas and a gas having molecular oxygen in a varying ratio is passed through the vacuum chamber. The input voltage is plotted relative to the input power to the cobalt target. Then above a certain input power, there appears a transition point at which the input voltage abruptly rises. This transition point varies depending on the ratio of inert gas to oxygen, pressure and other sputtering conditions. Independent of sputtering conditions, films formed with an input power which is equal to or higher than the transition point are oxidized to an adequate degree. Therefore, a bond exhibiting improved wet heat resistance is achieved without post treatment or heat treatment. A similar transition point is observed when a sputtering apparatus designed to deposit films in a different manner is used. A cobalt oxide film formed with an input power at or above the transition point is improved in resistance to wet heat deterioration after rubber is vulcanized thereto.
In a first aspect, the present invention provides a method for forming a thin film of a metal compound on a substrate, comprising the step of sputtering a target of a corresponding metal or metal compound in the presence of an inert gas and a reactive gas capable of reacting with atoms sputtered from the target to form the metal compound. The method further includes the steps of previously determining from the relationship of an input power supplied from a DC supply to the target to an input voltage between the target and the substrate during provisional sputtering at a predetermined vacuum and in a predetermined ratio of the reactive gas to the inert gas, a transition point of input power at which an abrupt change of the input voltage occurs, and controlling the input power during sputtering using the transition point as a reference, thereby controlling the composition or physical properties of the metal compound thin film. In one form, the input power is below the transition point. In another form, the input power is at or above the transition point.
In preferred embodiments wherein the metal compound is a metal oxide MeOx, metal nitride MeNx or metal carbide MeCx wherein Me is a metal atom, the input power is controlled to be below or at least the transition point, thereby controlling the value of x.
In a second aspect, the invention provides a method for preparing a rubber composite material comprising the steps of forming a thin film of cobalt oxide on a surface of a substrate, forming a rubber composition on the thin film, and vulcanizing the rubber composition. The step of forming a thin film of cobalt oxide includes the steps of sputtering a target of cobalt in the presence of an inert gas and a gas having molecular oxygen with an input power which is at least the transition point at which an input voltage between the target and the substrate abruptly rises when an input power is supplied to the target from a DC supply.